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The proper operation of the grid-connected power electronics converters usually needs using some kind of synchronization technique in order to estimate the phase of the grid voltage. The performance of this synchronization technique is of a great importance when trying to improve the quality of the consumed or delivered electric power. The synchronous reference frame phase-locked loop (SRF-PLL) synchronization algorithm has been widely used in recent years due to its ease of operation and robust behavior. However, the estimated phase can have a considerable amount of unwanted ripple if the grid voltage disturbances (e.g. harmonic distortion and unbalance) are not properly rejected. The aim of this paper is to propose an adaptive SRF-PLL which is able to strongly reject the aforementioned disturbances even if the fundamental frequency of the grid voltage varies. This synchronization method will allow the designer to easily upgrade an existing SRF-PLL, thus improving the performance of working power converters. This is accomplished by using several adaptive Infinite Impulse Response (IIR) notch filters, implemented by means of an inherently stable Schur-lattice structure. Besides the stability properties, this structure accomplished the most important topics required to be programed into the commonly used fixed point DSPs (i.e. high mapping precision, low round-off accumulation, suppression of quantization limit cycle oscillations). The proposed adaptive SRF-PLL has been tested by programing the algorithm into the fixed-point digital signal processor TI TMS320F2812. The obtained experimental results show up that the proposed synchronization method highly rejects the undesired harmonics, even if the fundamental harmonic frequency of a highly polluted grid voltage abruptly varies.

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Operation of doubly fed induction generators subjected to transient unbalanced voltage dips is analyzed in this article to verify the fulfillment of the Spanish grid code. Akagi's p-q theory is not used for this study, because control of the electronic converter is not the main goal of the paper, but rather to know the physical phenomena involved in the wind turbine when voltage dips occur. Hence, the magnetizing reactive power of the induction generators and their components, which are related with the magnetic fields and determine operation of these machines, are expressed through the reactive power formulations established in the technical literature by three well-known approaches: the delayed voltage (DV) method, Czarnecki's Current's Physical Components (CPC) theory and Emanuel's approach. Non-fundamental and negative-sequence components of the magnetizing reactive power are respectively established to define the effects of the distortion and voltage imbalances on the magnetic fields and electromagnetic torques. Also, fundamental-frequency positive-sequence and negative-sequence reactive powers are decomposed into two components: due to the reactive loads and caused by the imbalances. This decomposition provides additional information about the effects of the imbalances on the main magnetic field and electromagnetic torque of the induction generator. All the above mentioned reactive powers are finally applied to one actual wind turbine subjected to a two-phase voltage dip in order to explain its operation under such transient conditions. OPEN ACCESSEnergies 2011, 4 1149

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